1. Field of the Invention
This disclosure relates generally to white light-emitting devices using a semiconductor as a light source and a luminescent wavelength converting component, which devices provide reduced chromatic variation and improved luminescent efficiency.
2. Description of the Related Art
Solid state light-emitting devices such as light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs) sometimes called organic electroluminescent devices (OELs), and inorganic electroluminescent devices (IELs) have been widely utilized for various applications such as flat panel displays, indicators for various instruments, signboards, and ornamental illuminations, etc. As the emission efficiency of these light-emitting devices continues to improve, applications that require much higher luminance intensity, such as automobile headlights and general lighting, may soon become feasible. Among these applications, white LED is one of the promising candidates and has attracted much attention.
Conventional white LED's are manufactured based on a combination of a blue LED and yellow light-emitting YAG:Ce phosphor powder used as a wavelength-converting material dispersed in an encapsulant resin such as epoxy and silicone, as disclosed in U.S. Pat. No. 5,998,925 and U.S. Pat. No. 6,069,440. The wavelength-converting material is so disposed as to absorb some part of the blue LED light-emission and re-emit the light at a different wavelength as yellow or green-yellow light. The combination of the blue light from the LED and the green-yellow light from the phosphor results in perceived white light. A typical device structure is shown in FIGS. 1A and 1B. A submount 10 shown in FIG. 1A has a blue LED 11 mounted thereon and encapsulated by a protective resin 15. As shown in FIG. 1B, the blue LED 11 is covered with a transparent matrix 13, often constituted by silicon resin, in which YAG:Ce phosphor powder 12 is disposed. However, as shown in FIG. 2, since the particle size of YAG:Ce phosphor powder utilized for this system is around 1-10 μm, the YAG:Ce powder 12 dispersed in the transparent matrix 13 can cause strong light scattering. As a result, a considerable portion of both incident light 18 from the blue LED 11 and yellow light 19 emitted from the YAG:Ce powder ends up being backscattered and dissipated, causing a loss of white light emission.
One solution to this problem is to form a monolithic ceramic member as a wavelength-converting material. Ceramic bodies have been disposed in the path of light emitted by a light source as shown in U.S. Pat. Nos. 7,361,938 and 7,554,258. The ceramic member can be spaced apart from or abutting the LED light source as shown in U.S. Pat. No. 7,554,258. However, greater market demands are being made for reduced color binning variance and higher luminescent performance.
In addition, some LED constructs utilize p- and n-type contacts (flip-chip led light sources) (as shown in U.S. Pat. Nos. 7,521,862, and 7,402,840). These constructs can have ceramic bodies with about the same linear dimensions or surface area as the LED light source, wherein the ceramic bodies are spaced from but substantially radiationally in contact with the LED light source. However, these types of constructs do not utilize wire contacts and thus do not require a spaced apart relationship between the LED light source and the ceramic bodies to provide a passage for the connecting wires. Nevertheless, embodiments with p- and n-type contacts have described distancing the ceramic bodies from the light source (as shown in U.S. Pat. No. 7,070,300).
The manufacture of LEDs has also increased the demand for better products, and thus a reduction of color variability within a plurality of manufactured LEDs. U.S. Pat. No. 7,344,952 describes a luminescent encapsulation film disposed over an array of LED light sources. U.S. Pat. No. 7,294,861 describes an LED array using phosphor tape. However none of these attempts have provided an LED construct that provides consistent color and reduced variability. U.S. Pat. No. 7,462,502 describes a method for color control of a light emitting device by altering the thickness or amount of the ceramic or light emitting member. However, where the emissive layer is a ceramic layer having a thickness on the order of 100 μm, altering the thickness of the layer to adjust the color is difficult. Several U.S. patents responded to this problem by providing cover members or filters to adjust the chromaticity of a device (as shown in U.S. Pat. Nos. 7,180,240 and 7,402,840). However, none of these attempts have considered a construction that, without the use of additional filters or cover members, provides consistent color and reduced variability.